fix endless recursion when using inspect to reimplement global tostring

spec/inspect_spec.lua

@@ -411,4 +411,19 @@ describe( 'inspect', function()
       end)
     end)
   end)
+
+  it('allows changing the global tostring', function()
+    local save = _G.tostring
+    _G.tostring = function(x) 
+                    if type(x) == "string" then 
+                      return x 
+                    else 
+                      return inspect(x) 
+                    end 
+                  end
+    local s = tostring({1, 2, 3})
+    _G.tostring = save    
+    assert.equals("{ 1, 2, 3 }", s)
+  end) 
+
 end)